Possible dates for the last common ancestor of living penguins have differed by tens of millions of years. According to DNA evidence, the early ancestor lived some 40 million years ago, while fossil evidence puts the date closer to 10 million years ago.

Now a new genetic study, detailed in this week's issue of the journal Biology Letters, suggests that today's major penguin lineages began diverging from one another about 11 to 16 million years ago, and that their common ancestor first appeared 20 million years ago.

The study also suggests that a prolonged cooling spell in Antarctica may have helped spur penguins to diversify into the 18 species living today.

The new finding, based on more DNA points than past studies,